A new review adds to mounting evidence that dietary supplements may do more harm than good.

Selenium, a mineral found in fish, meat, eggs, grains and certain nuts, helps boost antioxidant activity in the body to potentially ward off chronic diseases. But too much selenium can cause selenosis, a condition marked by hair and nail loss, garlic breath, fatigue and nerve damage.

The recommended daily intake of selenium is 60 micrograms for men and 53 micrograms for women, according to the review published today in The Lancet. But men and women in the U.S., where 50 percent of the population takes supplements, get an average 134 and 93 micrograms per day, respectively.

"Additional selenium intake may well benefit people with low [selenium status]," Margaret Rayman, a professor of nutritional medicine at the University of Surrey in the U.K., wrote in the review. "However, people of adequate or high status could be affected adversely and should not take selenium supplements."

Adults should not consume more than 400 micrograms of selenium per day, according to the  National Institutes of Health Office of Dietary Supplements. Some selenium supplements contain 200 micrograms. And the  daily multivitamin Centrum contains 55 micrograms, enough to exceed the recommended daily intake in a person who gets sufficient selenium from a balanced diet.

"People who take dietary supplements are the least likely to need them," said Dr. Donald Hensrud, associate professor of preventive medicine and nutrition at the Mayo Clinic in Rochester, Minn. "People who have a bad diet typically can't afford them or tend not to take them."

Hensrud credited "Mother Nature" with making the concentrations of vitamins and minerals in fruits, vegetables, whole grains and lean sources of protein "optimal for health."

"We like to believe if we can concentrate something and put it in pill form, it must be good for us," he said. "And supplements are a multibillion dollar industry, so this belief is fueled by marketing and advertising."

But some people have difficulty getting enough selenium, despite a healthy diet. People with Crohn's disease and other gastrointestinal disorders, as well as people with iodide deficiency,  may need to take selenium supplements, according to the NIH.

"Obviously, if someone has a deficiency then they do need a supplement," said Hensrud.

Some studies have suggested selenium supplements can reduce the risk of cancer, cardiovascular disease and type 2 diabetes. But Rayman said more research is needed to tease out the mineral's benefits and risks.

"The effects of selenium on human health are multiple and complex, necessitating further research to optimize the benefits and reduce the risks of this potent trace mineral," she wrote.